Three current sensors are generally used to measure currents in each phase of a three-phase motor. The cost of such sensors and related conditioning circuitry adds cost to the motor drive circuitry. The current sensors also take up additional space, making the overall size of the drive circuitry of the motor larger.
A single current sensor can be used for a three-phase motor when individual phase current information and accuracy is not required or needed. If individual phase current information is needed, more complex control methods and systems are employed with sensors for each of the individual phases. One method of using a single current sensor, as described in U.S. Pat. No. 7,193,388, involves shifting or offsetting the duty cycle times 120 degrees continuously in time and adjusting the analog/digital (A/D) trigger or sample time based on the pre-set offset or phase-shifted times. However, shifting the signals by such large amounts may cause undesired distortion which must be corrected or filtered out and/or may cause other timing conflicts, such that additional shifting other than the preset offset of 120 degrees may be required.
Accordingly, there is a need for improved methods of single-shunt current sensing.